This invention relates generally to an apparatus and method for nondestructively testing bonded laminar or honeycomb structures, and more particularly to a device and method for accomplishing such testing for both detection and diagnosis of bondline conditions in such structures.
It has become increasingly more common, particularly in the aircraft and space vehicle industry, to fabricate structures comprising laminated and honeycomb panels having a plurality of bonded layers of lightweight high strength materials. Such panels have multiple layers bonded together to provide sufficient strength and resistance to the otherwise destructive effects of vibration and environment to which aircraft and space vehicles are normally subjected. Consequently, it is imperative that such structural bonds be tested to determine whether or not any flaws or substandard conditions exist, and the nature of such flaws and their locations within the bonded structure.
Numerous prior art devices and methods provide means for detecting the existence of unbonds in bonded structures. For example, U.S. Pat. No. 3,453,872 to Botsco, discloses an eddy sonic inspection method which relates to inspection of panels of sandwich-type construction, by progressively exposing one surface of the panel to a fluctuating magnetic field which penetrates the panel and results in acoustical variations caused by defects. In another eddy-sonic U.S. Pat. No. 3,564,903, to Woodmansee et al, to detect flaws in a laminated structure, a magnetic transducer causes the panel to acoustically vibrate at two times the exitation frequency of energy applied to a probe. A vibration detecting microphone, mounted directly to the panel under test, utilizes frequency filtering to provide means for indicating a signal change which represents a flaw in a bond in the material under test.
Other means for non-destructive testing of bonded structures for the purpose of detecting flaws in the bonds between the layers of the laminate, are described in a two part article by R. J. Schliekelmann in the periodical "Non-Destructive Testing", April 1972, page 79 through 84 and June 1972, page 144 through page 153.
However, in all such prior art disclosures of apparatus and methods for non-destructive testing of bonded structures, one disadvantage remains, namely, that of having no convenient way of diagnosing such flaws to precisely determine their location and also the nature of the bondline condition. The present invention combines phase and amplitude information derived from a signal responsive to the presence of a detected flaw or bondline condition in a structure, but it does so in a unique manner not heretofore known in the prior art. Although some of the aforesaid disclosures of prior art instruments and methods describe the use of the phase and amplitude of a flaw responsive test signal, none describes a method or apparatus for utilizing combined phase and amplitude information in the unique manner of the present invention, namely, the presentation of the equivalent vector on a complex impedance plane display which conveniently indicates the location and type of flaw that has been detected.